Method of manufacture of paper planographic plate



. tte

This invention relates to the coating of paper sheets and webs and particularly to the making of paper base planegraphic printing plates by coating procedures.

In particular, the coating of paper to serve as a pianograp-hic printing member has been conducted by providing on a paper base a water insoluble coating of certain suitable film formers which are hydrophilic, out which are also ink receptive when dry. The paper bases usually employed are impregnated or have otherwise incorporated in their makeup suitable wet strength imparting ingredicuts, for example, melamine-formaldehyde condensation products Other coatings of various materials are also often applied to one or both sides of the base sheet before the surface printing coat is applied for various purposes, one important use of such coatings being to prevent or delay the absorption of water by the paper foundation and lengthen the life of the plate. An example of paper planographic plates made generally according to such procedures is to be found in my Patent No. 2,65 5,864.

in carrying such manufacturing procedures into operation it has been discovered that the results in many cases, while giving printed copies of acceptable quality, were not acceptable for the reason that the sheets often had a strong tendency to curl which made them difficult to handle, store, and use. This curling tendency was not uniformly prescut, so that the reasons for it and the method for correcting it have not been heretofore ascertained.

It is the object of the present invention, therefore, to provide a simple readily controllable method for producing planographic printing or master sheets of the coated paper type which are fiat when completed and ready for use, and to provide the improved curl-free paper printing plates which result from this method.

in processing the paper plates of the type with which this invention is concerned it was found that the curl tendency was always towards the coated or printing face of the sheet, and it was further discovered that the primary curl-inducing factor was the treatment of the printing surface coating to render thesame water insoluble. This treatment of the film-former of the printing surface coat is carried out with various reagents which may be of various chemical makeup. in the form illustrated in said Patent No. 2,655,864, the reagents are water soluble salts of various polyvalent metals which have a coagulating effeet on the film-forming material of the printiuglayert Whether the reaction with the salts causes the layer to shrink parallel to the paper surface is not definitely known, but it has been quite conclusively determined that it is the salt application which accounts for the major portion of this pronounced tendency to curl since unsalted Plates are largely free of this tendency.

It has ben discovered, according to the present invention, that similar shrinkage or curl-provoking reactions can be brought about with most film formers capable of coagulation and used for coating paper, whether or not they are suitable for printing surfaces or are preliminarily Water insoluble, and that by treating the coated back surface of the plate with a salt solution somewhat similar to that used on the front or printing surface of the plate, the curl effect can be balanced out to produce a substantially flat sheet.

Another discovery made as a part of the present invenlot:

tion is that the order of the coagulation treatments has an effect upon the ultimate curl tendency and that, other things being equal, the surface to which the lastcoagulation treatment, including drying, was applied is the one towards which the sheet most frequently tends to curl. It is an object of the present invention, therefore, to provide a method whereby the time factor can be removed by applying the coagulation treatments to the surfaces and drying them substantially simultaneously with added assurance that the curling tendency will be removed.

In general it has been found that where coatings are similar, coating weights on both surfaces are about the same, and the salts or other coagulants applied are in substantially equivalent concentrations in their solutions, freedom from curl is assured. This, however, is an unusual and often impractical situation for many reasons, and it has also been found that substantial variations from the equality of these factors can be indulged in without seriously affecting thernon-curl quality of the sheet, so' long as both surfaces are treated, and especially when both are treated simultaneously. For example, it has been found possible to apply the invention very conveniently to situations where one surface, for example the back or secondary surface, is much more lightly coated than the other, or primary surface. Similarly, where the coating materials used are different or where the salts or coagulating materials used are different, there may be some difference in the power of the counteractive curl tendency introduced. It has been discovered that any of these factors are readily accounted for by the simple expedient of adjusting the concentration of the coagulating solution applied to the back or secondary surface, lighter coatings tending to cause less counteractive curl for a given salt concentration, and heavier coatings more counteractive curl. Thus by relatively simple preliminary tests, based on the film formers to be used, it is possible to work out a value of salt concentration, or a table of values, such that secondary surface treatments based on these values will produce a counteractive curl tendency which exactly or substantially overcomes the curl tendency of the prescribedtreatments on the primary or printing surface, resulting in a sheet which for all practical purposes is flat and will remain so. 7

It has further been discovered as a part of this invention that the action of coagulating reagents, especially the polyvalent salts of such mineral acids as N SO and HCl, has an additional valuable curl-preventing effect previously unrecognized. This relates to the paper foundation sheet itself Without reference to its coatings. Such a simple paper sheet is known to have different surfaces due to its process of manufacture, one being known as the wire side, and the other the felt side. Due-to differences of absorptivity these tend to absorb moisture in different degrees so that the sheet will have varying degrees of curl depending upon the prevailing moisture conditions in the air. It has been discovered, however, that treatment of the surface of the sheet with the coagulants mentioned above has an effect which largely eliminates this variable moisture curl tendency, apparently by so aifecting the surface fibers of the-sheet that they lose a largepart of their absorbency.

While the effect referred to in the immediately foregoing paragraph is, of course, of little importance Where both slides of the sheet are heavily protected with waterproofing or moisture barrier coatings, as is the case with certain planographic printing plates, in many cases the coatings do not fully exclude moisture from the back or secondary surface of the sheet, so that variable moisture curl problems can be encountered to a material extent. In such a case, treatment of the sheet with the coagulant salts has a pronounced effect in reducing this type of curling tendency also.

tion member for a planographic printing plate is coated,

by hand or using a lab coater, on one surface only with a Water dispersion of a colloidal film-forming material having the hydrophilic-oleophilic balance necessary to the formation of a planographic printing surface. The actual formula which is to be later used in machine coating the printing surfaces of plates in quantities will be used at this time. A variety of colloidal materials which have been discovered to have the necessary properties may be used, but for the purposes of this example, the material is taken as a carboxymethyl cellulose compound as set forth in said patent. An exemplary coating suitable for this purpose may be prepared by mixing the following ingredients to form a uniform slurry.

Formula I Ingredient: Parts by Weight aqueous solution of sodium carboxymethyl cellulose 100 China clay 20 Glycerine 3 Formula II Percent Copper sulphate 3 Aluminum sulphate 1 Ferric chloride /2 Chromium trioxide /2 as set forth in said patent. dried.

The resulting sheet will show a pronounced tendency to curl, and may now be used as a test sheet to ascertain the treatment which will, according to the present invention, be applied to the secondary or non-printing surface to prevent such curl.

.The principles normally governing the application of the coating to the secondary surface are that it be coagulable, form a smooth, continuous, flexible, dry, adherent coating, and be relatively inexpensive. Various colloidal materials such as casein, starch, soy bean flour, and alginic acid or alginates, will meet these requirements, but for the purposes of this example, it is proposed to use casein as being outstanding in all of the foregoing properties.

The sheet is then thoroughly An aqueous slurry of casein is prepared (Formula III) including about 5% of casein and coated on the secondary surface of the test sheet, the same then being substantially dried. By experiment the casein concentration can be adjusted to provide a continuous coating of minimum weight was to represent the most economical manufacture. About 5% as indicated is presumed to be suitable in most instances and results in a dry coating of about 4 lbs. per 500 ream of sheets 25" x 38".

The test sheet is then cut into a plurality of strips, five for example, which will be differently treated. A similar number of test solutions of the coagulant material are then prepared. For this purpose a wide variety of coagulant salts may be used, but for reasons of simplicity printing plate size.

and economy, alum will be selected as especially representative. The solutions may be prepared as follows:

Formula IV: 3 aqueous alum solution Formula V: 6% aqueous alum solution Formula VI: 9% aqueous alum solution Formula VII: 12% aqueous alum solution Formula VIII: 15% aqueous alum solution Each of the test strips is then washed with a different one of Formulas IV to VIII and all are dried. By an inspection of the strips it is possible to determine which one has the least tendency to curl and thus which formula is the correct one for treating the secondary surface of the plates during a commercial run. If opposite curl tendencies are discovered in formulas of adjacent numbers, then some percentage value between these two can be selected with success.

In carrying the invention into practice a commercial run is instituted in which the coating of the paper web is carried out in a continuous process. The coating as adopted for the printing face is applied, e.g. Formula I, and the coating for the secondary face, e.g. Formula IE is also applied to the web by any suitable coating apparatus. These are substantially dried on the machine and then the coagulation treatments are applied to the web. To the printing or primary surface the selected coagulant solution, e.g. Formula II, is applied, and to the secondary surface the appropriate coagulant as determined by test, e.g. the appropriate one of Formulas IV to VIII, is applied. These coagulant applications are preferably made at or about the same location on the machine, and in any event, both are simultaneously dried to produce a web which can be chopped into sheets of The sheets thus formed will be substantially flat and will remain so during storage in spite of variations in moisture content of the air.

While the foregoing description sets forth a simplified plate structure with no separate water barrier coating, it will be understood that the invention can also be applied to structures of a more complex nature. If a water barrier coating is employed this usually underlies the printing surface coating and may also be applied to the back surface of the plate. In such a case, that portion of the water barrier coating which appears on the back or secondary surface of the sheet will usually serve as the coagulable coating for the herein described curl prevention treatment, and no additional coating for this purpose need be provided. Such Water barrier coatings are frequently primarily of casein, or of a mixture of casein with other ingredients and hence act with similar effect to the casein secondary surface coating previously described. It is, however, possible to effect the treatment as disclosed when any other coagulable colloid is used as a significant part of the water barrier coating, as will be readily understood. Under circumstances where the water barrier coating takes the place of the secondary surface coagulable coating, any tests made for determining suitable percentages for coagulant solutions will be made on test samples using the material of the water barrier coating as the coating for the secondary surface of the sample.

It will be noted that in the procedure for making plates as described above the coagulant washes are applied to the surfaces of the sheet at about the same time and especially that drying of the washes occurs simultaneously. While it is possible to employ the other features of the invention without such simultaneous treatment, this is found to be of foremost importance in getting the most reliable curl inhibiting results in the easiest and least complicated manner, and is accordingly regarded as an important part of the invention.

While the specific coagulators referred to in the fore going description have been primarily water soluble salts, i

as formaldehyde or glyoxal which have a coagulating reaction on certain colloidal materials.

Variations and modifications may be made within the scope of the claims and portions of the improvements may be used without others.

I claim:

1. The process of making a coated paper planographic plate which comprises coating one surface of a paper sheet with a film-forming planographic surfacing material having a curl producing tendency; coating the other surface of the sheet with an aqueous dispersion of a coagulable colloidal material; and treating the colloid with a coagulant solution whose concentration is adjusted to cause a counteractive curl substantially equal in effect to the curl produced by said curl-producing material, whereby a substantially flat sheet is produced.

2. The process of making a coated paper planographic plate which comprises coating a primary surface of the sheet with an aqueous dispersion of a coagulable colloidal planographic surfacing material; coating the secondary surface of the sheet with an aqueous dispersion of a coagulable colloidal material; and treating the colloid on the secondary surface with a coagulant solution whose concentration is adjusted to cause a counteractive curl substantially equal in effect to the curl produced by the material on the primary surface, whereby a substantially fiat sheet is produced.

3. The process of making a coated paper planographic plate which comprises coating a primary surface of a paper sheet with an aqueous dispersion of a coagulable colloidal planographic surfacing material; treating said surface with a coagulant for said material, and thereby tending to produce a curl in the sheet towards said surface; coating the secondary surface of the sheet with an aqueous dispersion of a coagulable colloidal material having a dry weight substantially less than that of the colloidal material on said primary surface; and treating the colloid with a coagulant solution whose concentration is adjusted to cause a counteractive curl substantially equal in effect to the curl produced by the material on the primary surface, whereby a substantially flat sheet is produced.

4. The process of making a coated paper planographic plate which comprises coating a primary surface of a paper sheet with an aqueous dispersion of a coagulable colloidal planographic surfacing material; treating Said surface with a coagulant for said material, and thereby tending to produce a curl in the sheet towards said surface; coating the secondary surface of the sheet with an aqueous dispersion of a coagulable colloidal material having a dry weight substantially less than that of the colloidal material on said primary surface; and treating the colloid with a coagulant solution whose concentration is substantially greater than that of the solution used on said primary surface and which is adjusted to cause a counteractive curl substantiallyequal in effect to the curl produced by the material on the primary surface, whereby a substantially flat sheet is produced.

5. The process of making a coated paper planographic plate which comprises coating a primary surface of a paper sheet with an aqueous dispersion of a coagulable colloidal planographic surfacing material; treating said surface with a coagulant for said material, and thereby tending to produce a curl in the sheet towards said surface; coating the secondary surface of the sheet with an aqueous dispersion of a coagulable colloidal material having a dry weight substantially less than that of the colloidal material on said primary surface; and treating the colloid with a coagulant solution whose concentration is substantially greater than that of the solution used on said primary surface and which is adjusted to cause a counteractive curl substantially equal in effect to the curl produced by the material on the primary surface, said solutions being substantially simultaneously applied to and dried on both surfaces of the sheet, whereby a substantially fiat sheet is produced.

6. The process of making a coated paper planographic plate which comprises coating a primary surface of a paper sheet with an aqueous dispersion of a coagulable colloidal planographic surfacing material; coating the sec- Ondarysurface of the sheet with an aqueous dispersion of a coagulable colloidal material; and simultaneously treating the colloidal material on the primary surface with a solution of one coagulant, and the collodial material on the secondary surface with a solution of a different coagulant, the concentration of the latter coagulant being so adjusted that in reacting with the particular colloid on the secondary surface it will cause a counteractive curl substantially equal in effect to the curl produced by the coagulated colloid material on the primary surface whereby a substantially flat sheet is produced.

References Cited in the file of this patent UNITED STATES PATENTS 2,154,219 Shepherd Apr. 11, 1939 2,332,169 Robbins Oct. 9, 1943 2,373,287 Bassist Apr. 10, 1945 2,502,783 Erickson Apr. 4, 1950 2,511,783 Newman June 13, 1950 2,542,784 Van Dusen Feb. 20, 1951 2,635,537 Worthen Apr. 21, 1953 2,655,101 Newman Oct. 13, 1953 2,655,864 Newman Oct. 20, 1953 2,681,617 Worthen et al June 22, 1954 

1. THE PROCESS OF MAKING A COATED PAPER PLANOGRAPHIC PLATE WHICH COMPRISES COATING ONE SURFACE OF A PAPER SHEET WITH A FILM-FORMING PLANOGRAPHIC SURFACING MATERIAL HAVING A CURL PRODUCING TENDENCY; COATING THE OTHER SURFACE OF THE SHEET WITH AN AQUEOUS DISPERSION OF A COAGULABLE COLLOIDAL MATERIAL: AND TREATING THE COLLOID WITH A COAGULANT SOLUTION WHOSE CONCENTRATION IS ADJUSTED TO CAUSE A COUNTERACTIVE CURL SUBSTANTIALLY EQUAL IN EFFECT TO THE CURL PRODUCED BY SAID CURL-PRODUCING MATERIAL, WHEREBY A SUBSTANTIALLY FLAT SHEET IS PRODUCED. 